Binomial expansion: ( a + b x ) n (a+bx)^n ( a + b x ) n In a nutshell You can use the formula for the expansion of ( 1 + x ) n (1+x)^n ( 1 + x ) n to compute the expansion of ( a + b x ) n (a+bx)^n ( a + b x ) n for any constants a a a and b b b . In the case that n n n is not a non-negative integer, this expansion will only be valid when ∣ x ∣ < ∣ a b ∣ |x| < \left| \dfrac{a}{b}\right| ∣ x ∣ < b a .
Using the expansion of ( 1 + x ) n (1+x)^n ( 1 + x ) n The expansion formula tells you that:
( 1 + x ) n = 1 + n x + n ( n − 1 ) 2 ! x 2 + ⋯ + ( n r ) x r + … (1+x)^n = 1 + nx + \dfrac{n(n-1)}{2!}x^2 + \dots + \binom{n}{r}x^r + \dots ( 1 + x ) n = 1 + n x + 2 ! n ( n − 1 ) x 2 + ⋯ + ( r n ) x r + …
This holds in general whenever n n n is a non-negative integer, as in these cases only finitely many binomial coefficients are nonzero, whereas when n n n is not a non-negative integer this produces an infinite series, and only holds whenever ∣ x ∣ < 1 |x| < 1 ∣ x ∣ < 1 .
You can use this to derive the binomial expansion for ( a + b x ) n (a+bx)^n ( a + b x ) n using this formula. If a = 0 a=0 a = 0 , then ( a + b x ) n = b n x n (a+bx)^n = b^nx^n ( a + b x ) n = b n x n , but otherwise:
( a + b x ) n = ( a ( 1 + b a x ) ) n = a n ( 1 + b a x ) n = a n [ 1 + n ( b a x ) + n ( n − 1 ) 2 ! ( b a x ) 2 + ⋯ + n ( n − 1 ) ( n − 2 ) . . . ( n − r + 1 ) r ! ( b a x ) n + … ] \begin{aligned}(a + bx)^n &= \left(a\left(1+\dfrac{b}{a}x\right)\right)^n\\&= a^n\left(1 + \dfrac{b}{a}x\right)^n \\&= a^n\left[1 + n\left(\dfrac{b}{a}x\right) + \dfrac{n(n-1)}{2!}\left(\dfrac{b}{a}x\right)^2 + \dots + \dfrac{n(n-1)(n-2)...(n-r+1)}{r!}\left(\dfrac{b}{a}x\right)^n + \dots\right]\end{aligned} ( a + b x ) n = ( a ( 1 + a b x ) ) n = a n ( 1 + a b x ) n = a n [ 1 + n ( a b x ) + 2 ! n ( n − 1 ) ( a b x ) 2 + ⋯ + r ! n ( n − 1 ) ( n − 2 ) ... ( n − r + 1 ) ( a b x ) n + … ]
This holds in general whenever n ∈ N n \in \mathbb{N} n ∈ N , and if n n n is any other real value this only holds when ∣ b a x ∣ < 1 \left|\dfrac{b}{a}x\right| < 1 a b x < 1 , i.e. when ∣ x ∣ < ∣ a b ∣ |x| < \left|\dfrac{a}{b}\right| ∣ x ∣ < b a .
Example 1 Find the first 4 4 4 terms in the expansion of 1 12 − 3 x \dfrac{1}{\sqrt{12 - 3x}} 12 − 3 x 1 , and determine the range of values of x x x for which this expansion holds.
Rearrange 1 12 − 3 x \dfrac{1}{\sqrt{12-3x}} 12 − 3 x 1 :
1 12 − 3 x = ( 12 − 3 x ) − 1 = ( 12 − 3 x ) − 1 2 = ( 12 ) − 1 2 ( 1 − 1 4 x ) − 1 2 \begin{aligned}\dfrac{1}{\sqrt{12 - 3x}} &= (\sqrt{12-3x})^{-1}\\&= \left(12-3x\right)^{-\dfrac12}\\&= (12)^{-\dfrac12}\left(1 - \dfrac14x\right)^{-\dfrac12}\end{aligned} 12 − 3 x 1 = ( 12 − 3 x ) − 1 = ( 12 − 3 x ) − 2 1 = ( 12 ) − 2 1 ( 1 − 4 1 x ) − 2 1
Compute the first 4 4 4 terms using the expansion of ( 1 − 1 4 x ) − 1 2 \left(1 - \dfrac14x\right)^{-\dfrac12} ( 1 − 4 1 x ) − 2 1 :
1 12 − 3 x = ( 12 ) − 1 2 ( 1 − 1 4 x ) − 1 2 = ( 12 ) − 1 2 [ 1 + ( − 1 2 ) ( − 1 4 x ) + ( − 1 2 ) ( − 3 2 ) 2 ! ( − 1 4 x ) 2 + ( − 1 2 ) ( − 3 2 ) ( − 5 2 ) 3 ! ( − 1 4 x ) 3 + … ] = ( 12 ) − 1 2 [ 1 + ( − 1 ) 2 2 × 4 x + ( − 1 ) 4 × 3 2 3 × 4 2 x 2 + ( − 1 ) 6 × 3 × 5 2 3 × 6 × 4 3 x 3 + … ] = 3 6 [ 1 + 1 8 x + 3 128 x 2 + 15 3072 x 3 + … ] = 3 6 + 3 48 x + 3 256 x 2 + 5 3 6144 x 3 + … \begin{aligned}\dfrac{1}{\sqrt{12-3x}} &= (12)^{-\dfrac12}\left(1-\dfrac14x\right)^{-\dfrac12}\\&=(12)^{-\dfrac12}\left[1 +\left(-\dfrac12\right)\left(- \dfrac14x\right) + \dfrac{\left(-\dfrac12\right) \left(-\dfrac32\right)}{2!}\left(-\dfrac14x\right)^2 + \dfrac{\left(-\dfrac12\right)\left(-\dfrac32\right)\left(-\dfrac52\right)}{3!}\left(-\dfrac14x\right)^3 + \dots\right]\\&= (12)^{-\dfrac12}\left[1 + \dfrac{(-1)^2}{2\times 4}x + \dfrac{(-1)^4\times3}{2^3 \times 4^2}x^2 + \dfrac{(-1)^6 \times 3 \times 5}{2^3 \times 6 \times 4^3}x^3 + \dots\right]\\&=\dfrac{\sqrt{3}}{6}\left[1 + \dfrac{1}{8}x + \dfrac{3}{128}x^2 + \dfrac{15}{3072}x^3 + \dots\right]\\&= \dfrac{\sqrt{3}}{6} + \dfrac{\sqrt{3}}{48}x + \dfrac{\sqrt{3}}{256}x^2 + \dfrac{5\sqrt{3}}{6144}x^3 + \dots\end{aligned} 12 − 3 x 1 = ( 12 ) − 2 1 ( 1 − 4 1 x ) − 2 1 = ( 12 ) − 2 1 1 + ( − 2 1 ) ( − 4 1 x ) + 2 ! ( − 2 1 ) ( − 2 3 ) ( − 4 1 x ) 2 + 3 ! ( − 2 1 ) ( − 2 3 ) ( − 2 5 ) ( − 4 1 x ) 3 + … = ( 12 ) − 2 1 [ 1 + 2 × 4 ( − 1 ) 2 x + 2 3 × 4 2 ( − 1 ) 4 × 3 x 2 + 2 3 × 6 × 4 3 ( − 1 ) 6 × 3 × 5 x 3 + … ] = 6 3 [ 1 + 8 1 x + 128 3 x 2 + 3072 15 x 3 + … ] = 6 3 + 48 3 x + 256 3 x 2 + 6144 5 3 x 3 + …
This expansion is valid when ∣ − 1 4 x ∣ < 1 \left|-\dfrac14x\right| < 1 − 4 1 x < 1 , i.e. when ∣ x ∣ < 4 |x| < 4 ∣ x ∣ < 4 .
Therefore, the first four terms of the expansion of 1 12 − 3 x \dfrac{1}{\sqrt{12-3x}} 12 − 3 x 1 are 3 6 + 3 48 x + 3 256 x 2 + 5 3 6144 x 3 + … ‾ \underline{\dfrac{\sqrt{3}}{6} + \dfrac{\sqrt{3}}{48}x + \dfrac{\sqrt{3}}{256}x^2 + \dfrac{5\sqrt{3}}{6144}x^3 + \dots} 6 3 + 48 3 x + 256 3 x 2 + 6144 5 3 x 3 + … , and the range of values of x x x for which this expansion holds is ∣ x ∣ < 4 ‾ \underline{|x|< 4} ∣ x ∣ < 4 .